The invention relates to perpendicular magnetic recording media, and more particularly, relates to a soft magnetic underlayer of such media.
Perpendicular magnetic recording systems have been developed for use in computer hard disc drives. A typical perpendicular recording head includes a trailing write pole, a leading return or opposing pole magnetically coupled to the write pole, and an electrically conductive magnetizing coil surrounding the yoke of the write pole. A perpendicular recording medium may include a hard magnetic recording layer and a soft magnetic underlayer, which provide a flux path from the trailing write pole to leading opposing pole of the writer.
To write to the magnetic recording medium, the recording head is separated from the magnetic recording medium by distance known as the flying height. The magnetic recording medium is moved past the recording head so that the recording head follows the tracks of the magnetic recording medium, with the magnetic recording medium first passing under the opposing pole and then passing under the write pole. Current is passed through the coil to create magnetic flux within the write pole. The magnetic flux passes from the write pole tip, through the hard magnetic recording track, into the soft underlayer, and across to the opposing pole.
In addition, the soft underlayer helps during the read operation. During the read back process, the soft underlayer produces the image of magnetic charges in the magnetically hard layer, effectively increasing the magnetic flux coming from the medium. This provides a higher playback signal.
Perpendicular recording designs have the potential to support much higher linear densities than conventional longitudinal designs due to a reduced demagnetizing field in the recording transitions. In addition, the described bilayer medium is used in perpendicular recording to allow increased efficiency of the recording head. The soft magnetic underlayer of the perpendicular recording medium forms inverse image charges and substantially magnifies both the write field during recording and the fringing field of the recorded transition during reproduction. The quality of the image, and therefore the effectiveness of the soft underlayer, depends upon the permeability of the soft underlayer. The write and fringing field both increase rapidly when the soft underlayer permeability increases in the range from 1 to 100. Once the soft underlayer permeability goes above 100, the effect of the permeability on the write and fringing field is marginal. Therefore, to provide high efficiency of the recording head, the soft underlayer efficiency should not be less than 100.
To support the high image efficiency, the soft underlayer should be in an unsaturated state. However, during recording a top portion of the soft underlayer is likely to be saturated. Therefore, thickness and magnetic saturation induction, BS, of the soft underlayer needs to be matched to appropriate parameters of the recording head. Magnetic saturation of the soft underlayer causing the permeability reduction will result in write field degradation. Therefore, the soft underlayer should be relatively thick and have a high magnetic saturation induction, e.g. BS greater than 1 Tesla.
However, one of the challenges of implementing perpendicular recording is to resolve the problem of soft underlayer noise. The noise may be caused by fringing fields generated by magnetic domains, or uncompensated magnetic charges, in the soft underlayer that can be sensed by the reader. If the magnetic domain distribution of such materials is not carefully controlled, very large fringing fields can introduce substantial amounts of noise in the read element. Not only can the reader sense the steady state distribution of magnetization in the soft underlayer, but it can also affect the distribution of magnetization in the soft underlayer, thus generating time dependent noise. Both types of noise should be minimized.
In addition, the soft underlayer may form stripe domains, which generate noticeable noise and considerably reduce the signal-to-noise ratio of the recording medium. These stripe domains in the soft underlayer can be suppressed by applying an in-plane bias field. The bias field increases the effective anisotropy field of the soft underlayer and prevents domain formation that results in a permeability decrease. Techniques, such as, for example, permanent magnet or antiferromagnetic exchange biasing, are used to form the in-plane bias field. The permanent magnet technique assumes an application of high coercively magnetic film that generates a strong bias-fringing field. The antiferromagnetic exchange technique is based on antiferromagnetic film use. The antiferromagnetic film is placed in direct contact with the ferromagnetic soft layer and forms antiferromagnetic exchange coupling between the layers. Both of these techniques have disadvantages, such as, for example, high coercivity and low thermal stability of the biased soft underlayer. In addition, the antiferromagnetic materials have low corrosion resistance and require high temperature annealing to form exchange coupling. To be maintained in the single domain state by means of the antiferromagnetic exchange coupling or permanent magnet bias, the soft underlayer should be relatively thin. However, the relatively thin soft underlayer may not be useable due to its possible saturation during recording. Increase of the soft underlayer coercivity may also be unacceptable due to an increase in noise in the recording medium.
There is identified a need for a perpendicular magnetic recording medium with a soft magnetic underlayer that overcomes limitations, disadvantages, or shortcomings of known perpendicular magnetic recording mediums.
The invention meets the identified need, as well as other needs, as will be more fully understood following a review of this specification and drawings.
In accordance with an aspect of the invention, a perpendicular magnetic recording medium comprises a hard magnetic recording layer and a soft magnetic underlayer under the magnetic recording layer. The soft magnetic underlayer comprises a laminated structure which includes a first magnetic soft layer, a first interface layer on the first magnetic soft layer, a second magnetic soft layer, a second interface layer on the second magnetic soft layer, and a non-magnetic coupling layer between the first interface layer and the second interface layer. The first and second magnetic soft layers are antiferromagnetically coupled to one another via predominantly exchange interaction through the non-magnetic coupling layer. In addition, the first and second interface layers increase the exchange coupling effect between the first and second magnetic soft layers. The perpendicular recording medium may comprise additional soft magnetic underlayers having the same or similar structure as the described soft magnetic underlayer.
In accordance with another aspect of the invention, a perpendicular magnetic recording medium comprises a hard magnetic recording layer and a laminated soft magnetic underlayer under the hard magnetic recording layer. The laminated soft magnetic underlayer comprises means for antiferromagnetically exchange coupling the laminations thereof to one another.
In accordance with yet another aspect of the invention, a laminated soft magnetic underlayer of a perpendicular magnetic recording medium comprises a first magnetic layer, a first interface layer on the first magnetic soft layer, a second magnetic soft layer, a second interface layer on the second magnetic soft layer, and a non-magnetic coupling layer between the first interface layer and the second interface layer.
In accordance with a further aspect of the invention, a magnetic disc drive storage system comprises a housing, a perpendicular magnetic recording medium positioned in the housing, and a movable recording head mounted in the housing adjacent the perpendicular magnetic recording medium. The perpendicular magnetic recording medium comprises a hard magnetic recording layer and a soft magnetic underlayer under the hard magnetic recording layer. The soft magnetic underlayer comprises a first magnetic soft layer, a first interface layer on the first magnetic soft layer, a second magnetic soft layer, a second interface layer on the second magnetic soft layer, and a non-magnetic coupling layer between the first interface layer and the second interface layer.
In accordance with an additional aspect of the invention, a method of making a laminated magnetically soft underlayer of a perpendicular magnetic recording medium includes depositing a first magnetic soft layer on a substrate, depositing a first interface layer on the first magnetic soft layer, depositing a non-magnetic coupling layer on the first interface layer, depositing a second interface layer on the non-magnetic coupling layer, and depositing a second magnetic soft layer on the second interface layer.